Opening devices of different types provided on a packaging container formed by packaging sheet material and adapted for containing substances such as foodstuffs, and in particular for containing liquid beverages such as juices, water, milk, wine, etc. are known.
One such opening device includes a perforation on a wall part of the packaging container and a plastics material lid device attached to the wall part and covering the perforation in a closed state. The lid device includes a circumferential base attached to the wall part, for example by means of adhesives, circumferentially surrounding the perforation, and a lid element hinged to the base. Once the lid element is opened, the perforation may be engaged and forced open so that the contents inside the packaging container may be dispensed therefrom.
Another known opening device includes an opening provided on a wall part of the packaging container and a plastics material lid device similar to the one described above attached to the wall part and covering the opening in a closed state. A sealing element such as an aluminium pull tab is connected to the wall part so as to cover the opening, and once the lid element has been opened, the sealing element may be removed to expose the opening for dispensing the contents of the packaging container.
Methods for providing the above opening devices on the packaging containers may include feeding a continuous web of packaging sheet material to a first station at which the perforation or opening is provided on the web, and to a second station at which the lid element is attached to the web for covering the perforation or opening. The aluminium pull tab, if provided i.e. for covering the opening, is attached in an intermediate step. After the opening devices are provided on the web, packaging containers from the web are formed, filled with product, and sealed, such that the opening devices are arranged on the finished container packagings for easy access. One example of a machine for forming, filling, and sealing packaging containers is the TB8 filling machine manufactured by Tetra Brik Packaging Systems of Modena, Italy.
As an alternative to the above opening devices, in which an already formed plastics material lid device is applied to the packaging sheet material, U.S. Pat. No. 4,725,213 (the disclosure of which is incorporated herein by reference) discloses a plastics material opening device which is injection moulded directly onto the sheet of packaging material. In particular, a pair of mould tools are moved into arrangement about a prestamped hole in the sheet, and heated thermoplastics material is injected through an injection passage defined between one of the mould tools and one surface of the sheet and extending along such surface of the sheet in order to form the opening device at the prestamped hole. The mould tools are shaped so that the formed opening device includes a pair of circumferential flange portions each of which is attached to the opposite surfaces of the sheet at the edge of the prestamped hole, thereby attaching the opening device to the sheet.
Although the method and apparatus for providing this latter type of opening device may be completely valid, there exists in the field of thermoplastics material injected opening devices the need for further improvements.
It has been found for example that there exists a problem of controlling the flow of the injected thermoplastics material into the moulding cavity formed by the moulding tools so as to provide a proper positioning of the portion of the sheet adjacent the prestamped opening with respect to the opening device. Generally the portion of the sheet of packaging material adjacent the opening has a very low stiffness and during the injection moulding process such sheet portion is subjected to thermoplastics material at elevated pressures and temperatures. As a consequence this sheet portion adjacent the opening has the undesirable tendency to bend and finally end up in random positions in its connection to the injected opening device. What is needed is a method for injection moulding a thermoplastics material opening device onto a packaging material sheet which allows to achieve uniformity and assuredness with regard to the correct positioning of the sheet with respect to the opening device.
The above described bending tendency of an uncontrolled sheet portion adjacent the opening may lead to a situation in which during the thermoplastics injection step, the edge at the opening of the sheet portion may be in substantial contact with one of the mould tools and consequently plastics material will not flow between such mould tool and the surface of the sheet portion in contact therewith. As a result, the edge of the opening will be sealed in a poorer manner than would be an opening edge in which the plastics material of the opening device extends at least for a certain portion along the surface of the portion of the sheet adjacent the opening. Generally, the packaging sheet material is formed of several layers including an inner paper layer and two outer synthetic plastics material coating layers, one of which is destined to form a product contact surface. Other possible layers such as printing ink layers, laminating layers and aluminium foil layers may be interposed between the paper layer and the outer coating layers. When the opening in the packaging sheet material is stamped, the internal layers of the packaging sheet are exposed at the edge of the opening, and unless such edge is adequately sealed by the injected thermoplastics material opening device, the packaged product may come into contact with the internal layers of the packaging sheet material possibly causing damage thereto. What is needed is a method for injection moulding a thermoplastics material opening device onto a packaging material sheet which allows to assure that an adequate seal is obtained by the injected opening device at the edge of the opening formed in the packaging material sheet.